It is known in relation to wind power installations for them to be equipped with a synchronous generator and to provide an intermediate dc voltage circuit and a pulse inverter connected on the output side thereof as a frequency converter, for the variable-speed operation of the synchronous generator.
FIG. 4 is a circuit diagram illustrating the principle of such a wind power installation, wherein a variable-speed synchronous generator directly driven by the rotor is provided with a frequency converter connected on the output side thereof. In the intermediate dc circuit, firstly the variable-frequency current generated by the generator is rectified and then it is fed into the main network by way of the frequency converter.
The design configuration permits a wide range of speeds of rotation as the intermediate dc circuit provides for complete decoupling of the generator and therewith the rotor speed, from the mains frequency. The wide speed range permits effective wind-controlled operation of the rotor so that, when the design configuration is appropriate, it is possible to achieve a perceptible increase in its aerodynamically governed supply of power. It is almost self-evident that this design totally eliminates the unpleasant dynamic properties that the synchronous generator has in the event of direct connection to the mains network.
Up to a few years ago, a serious objection to the `synchronous generator with intermediate dc circuit` system was the high level of costs and the poor overall level of electrical efficiency. Because all the electrical output flows by way of the converter, the level of efficiency in the case of old installations was basically lower than with the variable-speed generator arrangements which use the converter only in the rotor circuit current of an asynchronous generator. Modern converter technology however has made that objection substantially irrelevant. Nowadays rectifiers and converters are designed whose losses are extremely low so that the overall level of efficiency of that generator system is as in the case of double-feed asynchronous generators.
The variable-speed synchronous generator with intermediate dc circuit is therefore nowadays very widespread in wind power installation technology. In particular modern inverters have made a significant contribution in that respect. In that connection, troublesome harmonics are substantially eliminated with so-called `pulse width-modulated (pwm) inverters`. Known pwm-inverters have a constant switching frequency or pulse duty cycle (also referred to as pulse frequency or pulse repetition rate) and the desired sinusoidal form of the alternating current to be fed in is formed by way of the ratio of the switch-on and switch-off times of two switches S1 and S2. The pulse duty cycle within which the switches S1 and S2 are switched on and off respectively is constant, as mentioned, and limited by the power loss of the inverter. In known inverters, the losses can be up to 2% or more of the total electrical output power generated, and that can be considerable in the light of the high level of costs of a wind power installation.
If the switching frequency is reduced, the power loss can admittedly be minimised but that causes an increase in the content of troublesome harmonics. If the switching frequency is increased, the power loss rises, as mentioned, but then the harmonics are very substantially eliminated.
DE 32 04 266 discloses a process and an apparatus for the operation of a pulse inverter in which an ac voltage which is synchronous with the desired inverter output voltage is compared to a delta voltage and when the two voltages are identical a change-over switching signal for the inverter switches is produced. To increase the output voltage amplitude the ratio of the control voltage amplitude and the delta voltage amplitude is raised to an over-proportional value.
DE 32 07 440 discloses a process for optimising the voltage control of three-phase pulse inverters, in which a constant dc voltage is supplied, in particular by an intermediate circuit. To optimise the voltage control of the three-phase pulse inverter, that process provides for the production of switching patterns which permit continuous adjustment of the fundamental oscillation voltage with the minimum possible harmonics effect.
Finally, DE 32 30 055 discloses a control assembly for a pulse inverter for producing an output ac voltage with a reference frequency which is predetermined by a frequency control, and a reference amplitude which is predetermined by an amplitude control voltage. The control assembly makes it possible in a simple manner to predetermine for an inverter, an output voltage which is optimised in regard to voltage utilisation and harmonics content.
Therefore the object of the invention is to provide a pulse inverter for a wind power installation, which avoids the above-mentioned disadvantages and overall reduces the power loss with a minimum content of harmonics.